| 摘要 |
The device for precisely measuring a vertical line comprises a thread holder (3) hanging a plumb (1) with a thread (2) at its center (P1), and a measuring tool (7) in which two light sources (5,5') are mounted and two screens (6,6') having center points (K1, K2) are prepared at the corresponding opposite sides to the light sources. When the centers of both the upper member (G1) and the thread holder are concentric, the shadow of thread is obtained on the screens by the light from the photodiodes (D1, D2) which passes the lenses (L1, L2) via the slits (S1, S2) of the shading plate (T1, T2).
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